Polymer material

ABSTRACT

A polymer material for manufacturing arcing chambers for low-voltage switchgear is characterized by a thermoplastic polymer matrix of polyamide or polyolefin which contains a cellulose material sheathed by cured melamine-formaldehyde resin.

BACKGROUND OF THE INVENTION

The invention relates to a polymer material for manufacturing arcingchambers for low-voltage switchgear, as well as to a method formanufacturing the same.

In low-voltage switchgear, such as low-voltage power circuit-breakers,the switch-off arc is supposed to migrate from the point of origin,i.e., from the separating contacts, quickly into the quenching device,i.e., into the arc splitter plates. The properties of the material ofthe arc-quenching or arcing chamber have a considerable effect on thisarcing movement, particularly in the case of short-circuit breaking. Ithappens, namely, during arcing movement that gaseous products arereleased from the chamber material because of the briefly occurring higharc temperature. These gases are needed to quickly extinguish the arc ifthey have the necessary chemical composition and when they areformed--per switching operation--in a quantity that is suited foravoiding too high a pressure inside the switch.

Under the state of the art, one has generally used molded materialsbased on cellulose-fiber-filled melamine-formaldehyde resins tofabricate arcing chambers (see, e.g.: Hansj urgen Saechtling"Kunststoff-Taschenbuch" [Plastics Manual], 24th edition, Carl HanserPublishers, Munich 1989, p. 452). However, these are thermosettingmaterials, and--due to the long mold occupation time and the requiredpostcure--it is, therefore, complicated and expensive to manufacture thechambers, even though this is done analogously to injection molding.

The object of the invention is to specify a polymer material that willmake it possible to substantially shorten the cycle times required inthe manufacturing of arcing chambers for low-voltage switchgear.

SUMMARY OF THE INVENTION

This is achieved in accordance with the invention by a material having athermoplastic polymer matrix of polyamide or polyolefin which contains acellulose material sheathed by cured melamine-formaldehyde resin.

DETAILED DESCRIPTION OF THE INVENTION

The polymer material according to the invention is a thermoplasticmaterial, i.e., a thermoplastic compound. The advantages of thiscompound--in comparison to thermosetting compounds--lie in an improvedprocessibility. While, namely, a 75 s cycle must be observed whenworking with the injection molding of thermosetting material, cycletimes as low as 3 s can be attained with the thermoplastic compoundaccording to the invention. It is thus possible, then, to have asubstantially more economical manufacturing. Moreover, from anelectrical standpoint, the thermoplastic material is at least equal tothe thermosetting material; the mechanical properties are evenbetter--due to a greater flexibility.

The chamber material produced from the thermoplastic compound exhibitsbetter arc quenching properties than the material used in known methodsheretofore. It is also superior with regard to the switching capacity,since the compressive load is substantially lower (reduction in themaximum pressure by up to 50%).

In the case of the thermoplastic compound according to the invention.,the polymer matrix consists of polyamide or polyolefin; preference isgiven to polyamide. For cellulose material, which is advantageously usedin the form of fibers, one can use in particular, pure cellulose, wooddust, paper or rayon staple fiber; cellulose is preferred in this case.

The individual components of the thermoplastic compound are preferablyavailable in the following ratio: polymer, i.e., polyamide orpolyolefin, to melamine-formaldehyde resin/cellulose material: 6:1 to1:1, preferably 2:1; melamine-formaldehyde resin to cellulose material:3:2 to 1:2, preferably 1:1.

The polymer material is produced in accordance with the invention inthat cellulose material sheathed by melamine-formaldehyde resin andpolymer are mixed together homogeneously at temperatures above 150° C.The temperature is dependent in this case upon the processingtemperature of the polymer used. When working with polyamide, it can liebetween about 190° and 300° C.; when working with polyolefin it amounts,for example, to 150° C. (polyethylene) or 200° C. (polypropylene).

In developing the polymer material according to the invention, theproblem arose of being able to work in temperature-sensitive materialsinto the thermoplastic material non-destructively. Namely, it is notpossible per se to use cellulose in high-melting thermoplastic systems,temperatures of >150° C. being necessary during intermingling andprocessing, because of the thermal-oxidative sensitivity of thecellulose.

Now, the invention resolves this problem by sheathing the cellulosematerial in situ with a protective layer, which--given a higher thermalstress of short duration--prevents oxidation or decomposition. Thismakes it easily possible to work the cellulose into higher meltingthermoplastic materials, such as polyamide.

In manufacturing the polymer material according to the invention, onestarts out preferably from uncross-linked melamine-formaldehyde resin;generally, one uses a commercially available, pre-reactedmelamine-formaldehyde resin (in short melamine resin) that is filledwith cellulose. During the intermingling or mixing operation, themelamine resin molding material is cured in the liquefied polymermaterial, i.e., a cross-linking takes place which, in this manner,protects the cellulose material from a thermal-oxidative attack. Thecellulose-filled melamine resin molded material formed in this case,which is distributed homogeneously ill the polymer matrix, assumes thefunction of a filler at the same time; this is synonymous with amechanical reinforcement.

Another important advantage of the invention consists in that there isthe possibility of recycling. This applies both for the waste materialproduced as sprues, for example, when arcing chambers are manufacturedusing injection molding, as well as for the thermoplastic compound thatis injection-molded to form molded articles. Used molded articles can becomminuted, namely, and the material obtained can be used--directly ormixed with fresh material--to injection-mold new arcing chambers. Incomparison, the thermosetting material used under the state of the art,itself, cannot undergo a recycling operation, because it isthree-dimensionally cross-linked.

The invention will be elucidated in the following on the basis ofexemplary embodiments.

EXAMPLE 1

PA 6 (polyamide from ε-caprolactam) is mixed in a ratio of 2:1 witch anuncross-linked melamine resin (MF), which is filled with cellulose in aratio of 1:1. This mixture is introduced at a temperature of 230° to240° C. into a masticator and masticated for about 10 min. Theprocessing temperature of the PA 6 of about 240° C. causes theuncross-linked thermosetting material, i.e., the MF/cellulose compoundto become cross-linked, and the cellulose is sheathed at the same time.

The thermoplastic compound produced in this manner--composition: 66.6%polyamide, 16.7% melamine-formaldehyde resin, 16.7% cellulose--has goodarc quenching properties and can be injection-molded perfectly well(cycle time: about 25 s). The cellulose acts in this case both as awater reservoir for the arc quenching, as well as--together With thecross-linked melamine resin--as a filler for reinforcing the polyamide.

EXAMPLE 2

One proceeds comparably to Example 1, but an oppositely rotatingtwo-wave extruder is used. The period of dwell (reaction time) amountsin this case merely to 5 to 6 min., so that a continuous process ispossible. The thermoplastic compound obtained is completely cured.

EXAMPLE 3

One proceeds comparably to Example 1, PA 66 (polyamide from 1.6hexandiamine and adipic acid) being used as a polymer; the processingtemperature amounts to about 275° C. The thermoplastic compounddemonstrates a high level of long-term heat resistance.

EXAMPLE 4

In a two-wave extruder, PA 66 is melted down directly downstream fromthe filling zone in counterrotation at a temperature of 275° to 280° C.Immediately after that--in a ratio of 2:1 --a MF/cellulose compound isintroduced by means of a dosing device into the liquefied polyamidematerial. The melamine resin that is immediately cross-linked at thehigh temperature of about 280° C. thereby sheathes the cellulose andprotects it from thermal decomposition. During a speed-dependent periodof dwell of 4 to 8 min, one achieves an adequate cross-linkage of themelamine resin and a homogeneous distribution in the polymer matrix. Theliquefied material emerging from the nozzle tip of the extruder is drawnoff as a strand, cooled and granulated. The thermoplastic compoundproduced in this manner is homogeneous and can be injection-molded toform perfect molded articles at temperatures of 275° to 280° C.

EXAMPLE 5

To process thermosetting waste material on the basis of melamine resin,which is filled with the most widely varying cellulose-containingmaterials, PA 6 or PA 66 or also another polyamide, such as PA 12 (fromε-lauric lactam) or PA 610 (from 1.6-hexanediamine and sebacic acid),are melted down in a two-wave extruder, which is operated in synchronismor in counterrotation, and mixed by means of a dosing device with theground waste material in a ratio of 6:1 to 1:1; the particle size of thepulverulent waste material should be <0.1 mm. The thermoplastic compoundobtained is granulated and dried using a procedure comparable to Example4, and can then be directly injection-molded to form arcing chambers.

One can proceed in a corresponding manner With cellulose-containingurea-formaldehyde resins, which are then worked into polyolefin.

What is claimed is:
 1. A polymeric material for manufacturing arcingchambers for low-voltage switching devices, comprising a thermoplasticpolymer matrix of polyamide or polyolefin which contains a cellulosematerial sheathed by cured melamine-formaldehyde resin, wherein thepolymer and melamine-formaldehyde resin/cellulose material are presentin a ratio of 6:1 to 1:1.
 2. The polymeric material according to claim1, wherein said ratio is 2:1.
 3. The polymeric material according toclaim 1, wherein the ratio of melamine-formaldehyde resin to cellulosematerial is from 3:2 to 1:2.
 4. The polymeric material according toclaim 2, wherein the ratio of melamine-formaldehyde resin to cellulosematerial is from 3:2 to 1:2.
 5. The polymeric material according toclaim 1, wherein the cellulose material is present in a fiber form. 6.The polymeric material according to claim 2, wherein the cellulosematerial is present in a fiber form.
 7. The polymeric material accordingto claim 3, wherein the cellulose material is present in a fiber form.8. The polymeric material according to claim 4, wherein the cellulosematerial is present in a fiber form.
 9. A method for manufacturing thepolymeric material of claim 1, comprising the step of homogeneouslymixing cellulose material sheathed by melamine-formaldehyde resintogether with polyamide or polyolefin at a temperature above 150° C. 10.The method according to claim 9, wherein the melamine-formaldehyde resinis not crosslinked.